The present invention relates to the magnetic resonance imaging and spectroscopy arts. It finds particular application in conjunction with dark blood flow tagging for angiographic imaging and will be described with particular reference thereto. However, it should be appreciated that the invention will also find application in connection with magnetic resonance excitation for other magnetic resonance applications.
It is well-known in the magnetic resonance arts that resonance can be excited in a planar region or slice by the simultaneous application of a selective RF pulse and static magnetic field gradient. This selective RF/static magnetic field gradient has been used to create localized excitation in a slice or planar region. This combination of selective RF pulse and static magnetic field gradient has been used in angiographic imaging for flow tagging. Typically, magnetic resonance was excited or saturated in a slice or slices adjacent a planar region or slice of interest. A series of RF pulses applied with a series of static gradients have been used to excite resonance or saturate blood in a plurality of slices or regions around the region of interest.
When a conventional imaging sequence was performed on the slice or other region of interest, the non-blood tissue was imaged normally. However, the blood which was tagged by prior excitation or saturation had different magnetic resonance imaging properties than blood which was not previously excited or saturated. As the blood from the adjacent slice(s) flows into the slice or region of interest, it changes the properties of the blood tissue displayed in the resultant image. This enables the resultant diagnostic images to be used to measure flow, measure flow rate, track flow paths, and the like.
One of the problems with tagging an entire region or plane is that it tagged not only the blood flow of interest but also blood flow which was not of interest. The plane typically extended across the examination region, tagging blood flow in portions of the patient that were not undergoing diagnostic examination. When the tagged blood from these remote regions flowed into the region of interest, the tagged blood of interest and the tagged blood from remote regions was indistinguishable. Accordingly, others sought to restrict the tagging region to a smaller region than the entire slice.
One technique for limiting the tagging region used a series of RF pulses with a series of different static magnetic field gradients to create the desired two-dimensional excitation profile. See for example, "Volume Selective Excitation: A Novel Approach to Topical NMR", W. P. Aue, S. Muller, T. A. Cross, and J. Seelig, J. Mag. Reson. Vol. 56, pp. 350-354, 1984; "Selective Spatial Presaturation of Regions of Tailored Shape", S. Singh, W. Brody, SMRM Book of Abstracts, 1992. These techniques generated a series of small RF pulses which summed together to produce the desired resonance excitation. The excitation region was rotated about the isocenter or other selected point such that there was a constructive superposition at a cylinder through the axis of rotation. The pulses were spread over the other remainder of the region providing negligible superpositions at other points, i.e., negligible resonance excitation.
One of the problems with the creation of a localized region of excitation by the superposition of RF pulses is that the technique is quite time consuming. Further gradient spoilers are commonly required between the individual RF pulses to insure that artifacts to not intrude in the image. The spoiler pulses increase the total time even more. Because the RF pulses were commonly identical and only the gradient direction was changed, excitation outside the volume of interest frequently occurred. Excitation outside the volume of interest could be suppressed by keeping the RF tip angle very low, but low tip angles increase the length of the pulse train even longer.
Other works used a single RF pulse in the presence of a time varying magnetic field gradient. See, "Off-Axis Spatial Localization of Frequency Modulated Nuclear Magnetic Resonance Rotating.rho. Pulses", C. J. Hardy, P. A. Bottomley, P. B. Roemer, J. Appl. Phys., Vol. 64, pp. 4741-4743, 1988; "K-Space Analysis of Small-Tip-Angle Excitation", J. Pauly, D. Nishimura, and A. Macovski, J. Mag. Reson., Vol. 81, pp. 43-56, 1989; U.S. Pat. No. 4,985,677 of J. Pauly; and U.S. Pat. No. 5,025,216 of Pauly and Nishimura. In these techniques, the intensity of the RF pulse and the intensity of the gradient field in two dimensions was combined to produce excitation which was localized in two dimensions.
By considering the RF and gradient coils together, a cleaner profile excitation was achieved, but at the expense of far more complicated radio frequency and gradient waveforms. The excitation was tailored to a specific point in space, typically the isocenter of the magnetic field gradients. Moving the region required recalculation of at least the phase profile of the radio frequency signal.
Others have combined portions of the two abovementioned techniques to decompose trajectories into a series of concentric circles or concentric squares. Other radial patterns, pinwheels, and Lissajous figures have also been used for excitation. These techniques require that suitable attention be paid to the homogeneous coverage of the frequencies of interest. See "Correcting for Non-Uniform K-Space Sampling in Two-Dimensional NMR Selective Excitation", C. J. Hardy, H. E. Cline, P. A. Bottomley, J. Mag. Reson., Vol. 87, pp. 639-645, 1990and U.S. Pat. No. 5,105,152 of J. Pauly.
These two techniques unified and optimized the RF requirements for the various trajectories. Identical RF pulses were provided for each spoke of a radial or pinwheel trajectory. Similar RF pulses were applied for each concentric square or circle. Again, the point of selective excitation was controlled by the phase of the RF pulses requiring recalculation of the RF pulse phase to shift the selective excitation region.
One problem in common with all of these techniques is that the region of interest was defined in only two dimensions. That is, the region of selected excitation was a cylinder which extended completely across the examination region along the third dimension.
In order to limit the region of selective excitation along the third direction, the Aue, et al. and the Crespigny, et al. articles suggested the creation of three-dimensional excitation profiles by a series of identical RF pulses applied in the presence of different magnetic gradient fields. This again required a relatively long duration because gradient ramping must occur between successive pulses. Further, these methods disturbed spins outside of the 3D volume, disturbing their equilibrium condition.
Another technique for limiting the field of excitation in three dimensions was described in "New Spatial Localization Method Using Pulse High-Order Field Gradients (SHOT: Selection with High-Order gradient)", C. H. Ooh, S. K. Hilal, Z. H. Cho, and I. K. Mun, Mag. Reson. Med., Vol. 18, pp. 63-70, 1991. This technique required high order, i.e. non-linear, pulsed magnetic field gradients to perform the volume selection. Selecting these gradients was again computationally intensive.
Another technique for limiting the excitation region was described in "A Three-Dimensional.pi.Pulse", J. Pauly, D. Nishimura, A. Macovski, SMRM 10th Annual Meeting, Book of Abstracts, Vol. 2, p. 493, 1991. This technique extended the two-dimensional pulse sequence of the previously discussed Pauly, Nishimura, and Macovski article in a third dimension but retained many of the drawbacks discussed above. Further, this technique required an inversion pulse in each repetition rather than an excitation pulse. That is, in order to control the localized excitation region in three dimensions, the prior art performed a succession of two-dimensional localization techniques in adjoining planes.
The present invention provides a new and improved magnetic resonance imaging and spectroscopy technique which facilitates limiting a region of excitation in three dimensions and which facilitates positioning a two or three-dimensional localized excitation region.